I just can't see how one works and the other doesn't, something fishy about that.

Which one isn't supposed to work?

As far as I know they all work to differing degrees. R value is one factor of course with foam winning over Reflectix. But the Reflectix is best at reducing the radiant. And the foam is best at the conductive.

If you are using only one make it closed cell foam. The Reflectix is an added bonus. This would be my choice for the hull. In the V-berth area I would insulate the most because that is where there is the most body heat near the hull sides when sleeping. My boat is getting a layer of Reflectix followed by 1/2" closed cell foam and then another layer of Reflectix, followed by wood trim in the V-berth area.

In the main cabin there are usually cabinets with dead space between the hull and any seating so a single layer of foam works well.

If you have to in some places use Reflectix by itself. This is not a bad choice for a location like the deck where the core is already acting as an insulator.

In all places whatever you use should be sealed well around the edges.